Transmission of data over long distances of optical fiber is limited by interference, such as from chromatic dispersion, which limits the usable bandwidth of the fiber. Chromatic dispersion is a result of the basic method by which fiber optic systems work. In particular, fiber optic transceivers work by transmitting “1” and “0” pulses, using two discrete levels of laser current to generate the two different pulses. But, the optical frequency of a semiconductor laser depends on the laser's current and the time derivative of the current. Ordinarily, this would not cause a problem, except that different optical frequencies travel at different velocities in an optical fiber. The result is that neighboring “1” and “0” pulses spread into one another. Over long distances of fiber, the effect from chromatic dispersion can become severe and the original data can no longer be recovered.
Chromatic dispersion is particularly an issue for 1550 nm laser light. This wavelength is used for long-distance transmission because it can be amplified by erbium-doped fiber or waveguide amplifiers, and because optical fiber has low attenuation at this wavelength. In contrast, laser light at 1310 nm, typically used for short-distance transmission, generally has very low chromatic dispersion in standard optical fiber.
Traditional prior-art solutions to chromatic dispersion generally fall into two classes: (i) limiting the optical frequency excursions (i.e., sidebands) of the transmitter, commonly known as “chirp”; and (ii) using special fiber- or optical-compensation elements so that different optical frequencies have the same transmit time from transmitter to receiver.
Examples of the specific technologies used to limit the chirp of a transmitter include: (1) special low-chirp or negative-chirp lasers that are designed to work at a fixed temperature maintained by a thermoelectric cooler; (2) externally-modulated lasers (EMLs); and (3) external modulation by lithium niobate or similar electrooptical modulators. However, these technologies generally add significant cost to a transceiver, as well as increased power consumption. Furthermore, it is theoretically impossible to completely remove chirp from a transmitter, since the modulation of an optical signal necessarily creates sidebands.
Examples of the specific technologies used as fiber- or optical-compensation elements include: (1) special low-dispersion fiber; (2) chirped fiber Bragg gratings; and (3) dispersion-compensating fiber. Like the technologies that limit chirp, these particular technologies are costly. They are also inconvenient for the customer to implement, and there is typically some residual dispersion penalty if the setup is not done perfectly.
A more advanced solution for addressing chromatic dispersion is described in U.S. Pat. No. 5,191,462 to Gitlin et al. In this patent, Gitlin describes a type of equalizer commonly used for fields such as voiceband data transmission, but adapted for the fiber optic transmission context. One of the key advantages of an electronic solution, such as in Gitlin, is reduced cost and complexity. Due to the inexpensive nature of electronic components, implementing signal processing techniques to compensate for dispersion in an optical data stream is highly desirable over the more expensive technologies described above.
The equalizer of Gitlin compensates for interference from dispersion by forming decisions as to a received signal (i.e., determining whether it is a binary “0” or “1”) by comparing the received signal against a special threshold value. The threshold value is determined from a feedback signal—specifically, a certain number of signals decided previously by the equalizer. The feedback signal directs a selection mechanism to pick the threshold value to be used from among a number of various threshold values. The various threshold values provided by the circuitry can either be predetermined, based upon expected fixed dispersion in the channel, or can be adaptive to changing dispersion.
Given the increasing speed of today's high-speed optical data communications system, with data rates of 5-10 Gb/s or even higher, and the increasing use of multiple channels on a single fiber optic, even more sophisticated techniques are desirable. Specifically, techniques that add minimal additional manufacturing costs, but which produce extremely reliable data recovery even when signals seem otherwise impossibly scrambled, would be highly desirable.